Move ESMValCore release documentation to How-to guide section

[jlenh]

Description

This PR adds a new "How-to" section in the documentation and moves the "making a release" section in a how-to guide in the newly created section.

Closes #2887

Link to documentation: https://esmvaltool--2889.org.readthedocs.build/projects/ESMValCore/en/2889/

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.63%. Comparing base (11ea6cb) to head (d708f69).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2889   +/-   ##
=======================================
  Coverage   95.63%   95.63%           
=======================================
  Files         263      263           
  Lines       15492    15492           
=======================================
  Hits        14816    14816           
  Misses        676      676           

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[ehogan]

Thanks @jlenh! 🥳

One very minor suggestion for you 😊

[ehogan]

Thanks @jlenh! 🎉

[valeriupredoi]

good to merge as soon as @bouweandela approves - he's got a 30min old review that needs addressing 🍺